Convert to for Processing (Credit Card Gateways Callback)
You have GBP 1.000 and EUR 1.190 currencies in your system and allow your customers choose the currency they want to pay in.
Under Credit Card gateway settings you set "Convert to for Processing: GBP" to ensure that all payments are sent to your GBP merchant account in proper currency.
Let's say your client attempts to pay 1.00 EUR invoice. WHMCS converts the amount accordingly and sends 0.84 GBP to the payment gateway according to conversion rate. So the payment is processed correctly. However it would add 0.84 EUR instead of 1.00 EUR to the invoice and the invoice remains unpaid. So the amount is not converted back upon the callback.
WHMCS advised that it's a common issue for any credit card gateway where the payment is processed without redirecting a client to the gateway. They also suggested that I throw in a feature request.
Please vote :)